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Aerobatics ServerACRO E-mail Archive Thread: [IAC] Correct snap roll catalogue and K-factor[International Aerobatic Club] [Articles etc] [Communications] [Contest and Patch Information] [Aviation Organizations] [Other Aerobatics Info] [Aerobatics Images] [Other Aerobatics Links] [Other Aviation Info (including weather)] [Search ACRO Website]Disclaimer: These aerobatics pages are developed by individual IAC members and do not represent official IAC policy or opinion. |
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Thread: [IAC] Correct snap roll catalogue and K-factor
Message: [IAC] Correct snap roll catalogue and K-factor
Follow-Up To: ACRO Email list (for List Members only)
From: Brian Howard <BK at NewAttAero.com>
Date: Tue, 13 Jun 2000 17:54:14 UTC
Competitors and Judges: Perhaps because I have reviewed more Unlimited and Advanced Frees than usual this year, I am seeing a greater number of cases where the incorrect snap roll catalogue number and K-factor have been used. This error results in the total K being under or over the max allowed and the designer is forced to go back to the drawing board with the Free. Never fun. As my recent relocation and schedule have put my HEADS UP! column on a temporary leave of absence, allow me to indulge in an electronic HEADS UP! column to review the process of choosing the correct snap roll catalogue number and K. This posting will be a little longer than your average, but because of virus fears (rightfully so) at the recipient's end, I chose not to make it an attachment. So, unless you are in the process of designing, or checking, a Free program right now, you may want to save this message and read it on one of your weather days. It is very important material to understand, however, and may save you a lot of grief redesigning your Free later. If you think using Aresti 5 saves you from this worry, read on! The primary way to pick the correct snap roll is to simply look at how the base figure is drawn in the FAI Catalogue. Note whether the snap is placed on a solid or dashed line within that figure, go to Family 9.9 or 9.10, as appropriate, in the Catalogue and use the snap depicted on the same line style, solid or dashed. If only that was all there was to it! Beyond looking at the line style on which the snap is located, there are five conditions which must be considered before finalizing your choice. 1. Is the snap located on the downline of a hammerhead? If so, you must use the lower K version of the snap. That is, a 9.9.5.X or a 9.10.5.X. 2. Is the snap located on the downline of a tailslide? If so, you must use the lower K version of the snap. That is, a 9.9.5.X or a 9.10.5.X. 3. Is the snap located after a spin element on a Family 1 or Family 8 figure? If so, you must use the lower K version of the snap. That is, a 9.9.5.X or a 9.10.5.X. 4. Does the snap FOLLOW an aileron roll (of any degree of rotation) on a vertical line, up or down? If so, you must use the lower K version of the snap. That is, a 9.9.1.X (up), 9.9.5.X (down) or a 9.10.1.X (up) or 9.10.5.X (down). If the snap roll PRECEDES the aileron roll, there is no exception and the loading of the aircraft (positive or negative) is used as the determiner, unless conditions #1 or #2 apply. Why didn't I include #3 as an exception? A maximum of two rotational elements are allowed on a line. Therefore, if you have a spin element, you could also have a snap element OR an aileron roll element, but NOT both. 5. This last condition takes the most thought and occurs with half-loops and anytime snaps follow other rolling elements (snap or aileron) on a line. The basic idea is stop and think about how the airplane is loaded AT THE INSTANT the snap would be initiated. Since I can't include graphics with IAC Exploder e-mail, you may want to consult your FAI Catalogue to follow these examples. Example 1: A 7.1.1, half inside loop up to inverted with a full positive snap at the top. Looking at the catalogue drawing, the half loop is a solid line followed by a dashed horizontal line at the top. When you draw the snap, it may appear to be located on the dashed top line of the loop and a 9.9.8.4 chosen. That would be INCORRECT. The snap is actually initiated the instant the half loop is complete, while the airplane is still under positive load (notice in the catalogue drawing that the optional roll symbol is connected to the solid, half-loop line). The correct snap is therefore a 9.9.3.4. Example 2: Same figure, but this time a full slow roll is inserted before the positive snap. There will be a brief, but perceptible pause between the rolls and to maintain level flight, the airplane must be negatively loaded (inverted). In this case therefore, the positive snap is initiated from a negative loading and the correct catalogue number is 9.9.8.4. One more just to make sure. Example 3: A 1.2.1, forty-five degree positive line up with a 1/2 positive snap followed by an opposite direction 1 1/2 positive snap. The first half snap is clearly drawn on a positive (solid) line, exceptions #1 through #4 don't apply, so no doubt in our minds that it is a 9.9.2.2. The following 1 1/2 positive snap is also clearly drawn on a positive line, but think. After the 1/2 snap, the aircraft is inverted (negatively loaded) on the 45-degree line, so even though the line is drawn solid, the loading is negative and a 9.9.7.6 snap must be used. What if you use the Aresti 5 software? How do you choose the proper snap and won't the Free Programme Tester add-on catch any mistakes anyway? When you drag a snap roll onto your drawing, Aresti 5 asks you for the snap type (positive or negative) and the line type (positive or negative). Choosing the snap type is obvious and so is the line type if none of our exceptions discussed above apply. If an exception does apply, simply remember that the lower K snap is always the one where the snap and line type are the same. If you are placing a negative snap on the downline of a hammerhead, for example, tell Aresti 5 that the snap is negative and the line type is negative (even though the hammerhead downline is drawn solid). IMPORTANT POINT: The Aresti 5 Free Programme Tester does NOT check for the proper snap roll catalogue number. It assumes you know the rules and chose the proper snap for the line loading. You must manually check each snap for the proper catalogue number. TIP: Besides looking at the Form A, if you have the SHAPE toolbar showing, you can quickly check the catalogue of each snap by selecting the snap element and then clicking the CUSTOM PROPERTIES button located at the far right side of that toolbar. As a competitor, learn how to choose the proper snap roll attitude and design your Free correctly to start with so you are not either forced into a redesign, or lose that piece of walnut you paid thousands of dollars to win. Judges, learn these rules so your name won't be on the Free which is declared illegal at a contest. Besides not being very flattering, you were entrusted by the competitor to check the Free for problems and you may have just cost that pilot a top finish or even a championship. For reference, choosing the correct snap roll attitude is discussed in both the IAC Rules Book, section 6.10, and the FAI Catalogue, paragraphs 23 and 24. If you made it this far, thank you for your attention! Brian Howard Editor, HEADS UP! Chairman, IAC Rules Committee
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